1. Field of the Invention
This invention relates to a method for the reuse of gallium nitride (GaN) epitaxial substrates.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Traditionally, (Al,In,Ga)N nitride devices have been grown heteroepitaxially on substrates such as sapphire and silicon carbide due to the lack of available bulk GaN substrates [1]. Non-native substrates have significant disadvantages, including lattice mismatch to GaN (which causes strain and deleterious defects such as threading dislocations) and the inability to grow high-quality GaN crystal films with orientations other than c-plane [1].
Non-c-plane orientations, including non-polar and semipolar orientations, exhibit reduced or eliminated internal electric fields in quantum well devices. This strain-induced piezoelectric polarization results in the quantum-confined Stark effect (QCSE), whereby a spatial separation between electron and hole wavefunctions reduces the recombination efficiency in light-emitting quantum wells [2]. The reduction or elimination of this field in non-polar and semi-polar crystal orientations can result in improved device performance [3], [4]. However, the lack of native bulk substrates has limited the application of these crystal orientations.
Recently, low-dislocation-density bulk GaN substrates have become commercially available [1], [5]. This has allowed for the realization of c-plane and non-c-plane devices with low threading dislocation (TD) densities [5]. It has also eliminated the lattice and thermal expansion coefficient mismatch problems that come with using non-native heteroepitaxy substrates. However, high cost has limited the adoption of bulk GaN substrates. Work continues towards bringing down the cost of bulk GaN substrates, a major barrier to widespread adoption of these preferential substrates [1], [5].
Substrate removal has previously been used in III-nitrides and other materials systems. For instance, GaN-based devices can be removed from sapphire substrates using laser liftoff. Flip-chip bonding has found application in technologies like processing of LEDs [6] and GaN power amplifiers [7]. However, it has not been feasible to remove bulk GaN substrates from homoepitaxially-grown films or devices. The ability to remove bulk GaN substrates from electronic and optoelectronic devices, without damaging the substrate itself, could allow for the reuse of these expensive substrates, substantially reducing the cost of individual devices.